A condenser microphone includes an impedance converter such as an FET (field-effect transistor) because a condenser microphone unit of the condenser microphone has quite a high impedance. In ordinary cases, a phantom power source is used in the condenser microphone, and the output of the microphone is outputted through a balanced shield cable of the microphone. FIG. 3 schematically shows the configuration of a microphone output module section used for a conventional condenser microphone.
A part indicated by a frame of a chain line with reference numeral 10 in FIG. 3 is a microphone case of the condenser microphone. The microphone case 10 also serves as a shield case and thus is made of a conductive metallic material such as brass. In the case of a handheld microphone, the microphone case 10 is used as a grip held by a hand of a person.
The microphone case 10 houses a circuit board 11 having a ground circuit 13 and an electronic circuit 12 which is connected to a condenser microphone unit MU and includes a lowcut filter circuit and an amplifier circuit (both are not shown). Further, an output connector 20 is provided on the microphone case 10.
In ordinary cases, the output connector 20 is a 3-pin output connector defined by EIAJ RC-5236 “a latch-lock round connector for an audio system.” To be specific, the output connector 20 comprises a first pin for grounding, a second pin used as the hot side of a signal, and a third pin used as the cold side of a signal (Reference numerals 1, 2, and 3 of FIG. 3 denote the first pin, the second pin, and the third pin, respectively) and the output connector 20 is connected to a phantom power source (not shown) via a microphone cable (balanced shield cable) 30.
Of these three pins, the second pin and third pin for signals are connected to the predetermined terminals of the electronic circuit 12 and the first pin for grounding is connected via a lead wire 1a to the microphone case 10 and the ground circuit 13 of the electronic circuit 12 formed on the circuit board 11. The lead wire 1a is routed in the microphone case 10.
As described above, the microphone case 10 is a shield case made of, for example, a metallic material such as brass. For example, when a cellular phone is used near the microphone, strong electromagnetic waves may enter the microphone case 10 from the microphone cable 30 through the output connector 20, and the electromagnetic waves may be demodulated by the electronic circuit 12 and outputted as audio-frequency noise from the microphone. Incidentally, extremely strong electromagnetic waves are generated from a cellular phone (for example, in a range of about several cm to several tens cm, an electric field is several tens of thousands times as strong as an electric field generated by commercial radio waves).
Regarding a noise generating mechanism, in Document 1, Jim Brown of Audio Systems Group Inc. of the US and David Josephson of Josephson Engineering of the US point out the following problems: (1) the lead wire 1a of the first grounding pin routed in the microphone case 10 acts as an antenna and draws high-frequency current of external electromagnetic waves into the microphone case 10 and (2) a stray capacitance C between the microphone case 10 and the ground circuit 13 formed on the circuit board 11 forms a ground loop current path (ground loop) as indicated by an arrow of FIG. 3, and Jim Brown et al. propose the following solution to noise:
In Document 1, Jim Brown et al. propose a method of connecting the ground circuit 13 formed on the circuit board 11 to the microphone case 10 via a proper wire 1b as shown in FIG. 4, and directly connecting the first grounding pin included in the output connector 20 to the microphone case 10 without connecting the first pin to the ground circuit 13.                [Document 1] “Radio Frequency Susceptibility of Capacitor Microphones,” cowritten by Jim Brown and David Josephson, Audio Engineering Society Convention Paper 5720 (page 12, FIG. 8).        
According to the method of Document 1, the stray capacitance C between the ground circuit 13 and the microphone case 10 does not form a ground loop current path and the lead wire 1a routed from the first grounding pin to the ground circuit 13 is not present, that is, nothing acts as an antenna. Thus, it is possible to effectively prevent the entry of electromagnetic waves.
However, in the case of the method described in Document 1, the first grounding pin is directly connected to the microphone case 10. Thus, when a phantom power source is used, current passes through the microphone case 10. Therefore, when the first grounding pin is detached from the microphone case 10 for any reason, the microphone case 10 has a voltage of 30 V or higher in the case of a 48-V phantom power source, and thus a person may receive an electric shock with a touch of a hand on the microphone case 10.